The conventional approach to electronic packaging and interconnect has been to package individual integrated circuit (IC) chips into a single package and to attach these packages to a printed circuit board to provide interconnect between the individual IC chips. In recent years, this approach has met with problems of speed, size and interconnect density due to ever-constant demands for reduced size and increased performance from such integrated circuit packages.
Process speed is conventionally limited by the fact that individual packages have loading capacitance and inductance associated with their relatively long pins and by the large size of conductor runs in the packages relative to the size of the conductors of a bare IC chip. As the speed of computers and other devices continues to increase, the problem of providing electronic packaging and interconnect which provides maximum performance has become a significant challenge. One solution to the problem is the multichip module (MCM). In an MCM, bare (unpackaged) IC chips are interconnected by means of a miniature printed circuit board whose interconnect pitch may match the pitch of the IC chips themselves. There are presently two main classes of MCM. These are the chips-last MCM and the chips-first MCM. In the chips-last MCM, the miniature circuit board is fabricated first and then the bare IC chips are attached and interconnected to the circuit board. The method of interconnect is usually wire-bond or solder bump. In the chips-first MCM, the chips are placed first relative to each other and a miniature circuit board is then built above the chips. The interconnect is formed to the IC chips as an integral part of the processing of the circuit board.
Structures in accordance with the present invention fall into the category of chips-first MCMs. Chips-first MCMs provide one way to minimize size of a multichip module and provide high performance interconnect. Examples of chips-first modules are given in U.S. Pat. Nos. 5,250,843; 5,353,498; and 5,841,193, each of which is hereby incorporated herein by reference in its entirety.
One problem with an assembly using chips-first technologies is that modules are costly to repair. If a module is fabricated with a defective integrated circuit chip, then all other chips in the module are lost. Thus, it is desirable to take advantage of a chips-first technology for size and performance aspects, while still providing a fabrication method which avoids incorporating components that are not thoroughly tested and known to be good.
Another problem in the art is to provide very thin chips-first modules that have mechanical rigidity and protection so that they are easily handled by automated assembly equipment and can be used without further structural considerations by, for example, a cell phone assembler. A further problem is that certain components of an electronic system, such as a cell phone, generate a significant amount of heat. An example of this is the power amplifier. The use of a heat sink is prohibitive in that it adds an unacceptable thickness to the package. Instead, the conventional approach is to deliver heat to the circuit board on which the components are mounted. Still another problem is that in an extremely dense package, cross-talk between sensitive RF receiver components, powerful transmitter components and high speed digital components is markedly increased. This problem is reduced in circuit board versions of a cell phone in that the individual components can be spaced from one another and individual shield shells placed around the cross-talking components.
The chips-first circuit structures and methods of fabrication presented herein are directed in part to addressing the above-noted problems.